Rewrite link parsing for finding v2 schemas

What we called 'links' are no longer returned in a container of
objects, they are top-level entity attribtues. This fixes the
parsing of the entities to look in the correct place when trying
to locate a specific schema.

Add a helper for printing to stderr and exiting with a non-zero
exit code.

Map 'name' to 'Attribute' when explaining a schema.

Related to bp glance-client-v2

Change-Id: Ib98e912a7af0bb570b4fd738733edd9b837d1a05

tests/v2/test_schemas.py

@@ -23,10 +23,10 @@ fixtures = {
     '/v2/schemas': {
         'GET': (
             {},
-            {'links': [
-                {'rel': 'image', 'href': '/v2/schemas/image'},
-                {'rel': 'access', 'href': '/v2/schemas/image/access'},
-            ]},
+            {
+                'image': '/v2/schemas/image',
+                'access': '/v2/schemas/image/access',
+            },
         ),
     },
     '/v2/schemas/image': {